A railway is a mode of transportation limited to one degree of freedom, meaning that a rail vehicle may only travel back and forth along a track and, absent the presence of a switch to change tracks, cannot alter its path to avoid other traffic. In order to prevent rail vehicles on the same track from running into one another, a block signaling scheme has been utilized whereby the track is divided into segments or “blocks.” Typically, wayside indicators placed before an upcoming block indicate to the locomotive operator whether or not an upcoming block is occupied by another rail vehicle, so that the speed of the operator's rail vehicle can be adjusted to avoid any potential interference between rail vehicles.
Railway signaling, such as to indicate a condition (e.g., the presence of a broken rail) or occupancy of a track block, has been traditionally controlled by a track circuit. The track circuit is essentially an electrical circuit in which the rails within a block provide electrical connection between an electrical signal transmitter and an electrical signal receiver. Electrical separation between adjacent blocks may be provided by insulating joints. The transmitter, such as a voltage source, impresses an electrical signal into the rails at one end of the block which may be received by a receiver, such as a relay, at the other end of the block. The relay can then operate to display an appropriate aspect on the wayside indicator.
Existing track circuit systems also provide means for coding the electrical signal to transmit and receive information through the rails of the track, as well as track occupancy detection by the shunting action of the wheels of the rail vehicle. These systems provide block occupancy information at both ends of the track circuit, as well as communicating occupancy in general through several track sections to a control point where the information may be transmitted to a central office for display.
In particular, codes are used to transmit information to a wayside signal. Typically, vital track codes are generated using one or more short, positive DC pulses on a DC circuit. The pulses are typically 80 to 250 milliseconds in duration with the pulses of multiple-pulse bursts being separated by brief intervals, on the order of 80 to 950 milliseconds.
With reference to FIG. 1, current coding schemes rely on a series of accurately timed, single direction, positive pulses on a DC track circuit. These pulses are generated by a transmitter at one end of a block, and are evaluated by a receiver at the other end of the block for timing to determine the particular code being transmitted. Adjacent blocks reverse the polarity of the track circuit connections so the entire coded signal is the reverse polarity, thereby enabling receivers to discriminate signals within their track block from signals in an adjacent track block.
Thus, with existing coding schemes, information is conveyed by pulse and separation widths. As shown in FIG. 1, for example, a first code 12 is defined by a single, short, positive DC pulse 14. A second code 16 is defined by a single, positive DC pulse 18 of slightly longer duration. A third code 20 is defined by a single, positive DC pulse of longer duration 22. As also shown therein, codes may also include multiple pulses. For example, a fourth code 24 may be defined by two short, positive DC pulses 26, 28 in quick succession, and a fifth code 30 may be defined by two short, positive DC pulses 32, 34 that have a longer time interval therebetween. As will be readily appreciated, however, existing coding schemes that convey information by pulse and separation widths are limited in the number of codes that that can communicate information within a defined code cycle.
As rail vehicle control has evolved, a need for additional information, such as speed restrictions or track conditions ahead, has grown. However, utilizing existing code generation and signaling systems and methods, the number of codes that can safely communicate information without making the code window longer, which would change the cycle time and delay detection and the relaying of data, is less than the number of conditions to be communicated.